1. Field of the Invention
The present invention relates to an edge-emitting type semiconductor laser in which a laser cavity and so on are formed by depositing plural number of semiconductor layers on a crystal growth substrate. The present invention is aimed to obtain a semiconductor laser having a completed far field pattern (FFP) of output light (beam) and output characteristic of high convergence.
So a semiconductor laser fabricated in the present invention is remarkably useful in a field of, for example, information input/output processing equipment, information calculation processing equipment, communication equipment, and a laser material processing equipment.
2. Description of the Related Art
[Non-patent Document 1]
SHARP Technical Journal No. 9, No. 77, “Transverse Modes of Ridge-Geometry Violet Laser Diodes”
[Patent Document 1]
Japanese Laid-open No. H10-308560
As disclosed in the above described non-patent document 1, as specific problems of semiconductor lasers, satellite peaks in vertical light distribution and pronounced ripples in the vertical far field patterns (FFPs) are found.
FIG. 7 illustrates a sectional view of a semiconductor laser, which explains the phenomenon of ripples occurred in the far field pattern (FFP). The ripples in the far field pattern (FFP) may also occur owing to lights leaked into and transmitted in an n-type contact layer functioning as a secondary (quasi) waveguide. Such lights which generate the ripples in the far field pattern (FFP), as disclosed in the above non-patent document 1, form interference fringes in the n-type contact layer and are radiated to the outside of the layer with lights radiated from a main waveguide (active layer+guide layer).
The conventional technique disclosed in the above non-patent document 1 is aimed to suppress the ripples in the far field pattern (FFP) by forming the n-type clad layer to have at least 0.8 μm or more in thickness.
Also, the above identified related patent document 1 discloses a concave part structure formed on the back surface of a crystal growth substrate of a light-emitting semiconductor device, external quantum efficiency, endurance and productivity of the device, and prima facie effectivity of the aspects can be recognized. The patent document 1, however, fails to disclose the optimum range of a specific thickness of a semiconductor layer as the above mentioned non-patent document 1 discloses. Accordingly, the patent document 1 never suggests a concrete and effective method or process for excluding the ripples occurred in the far field pattern (FFP), with respect to an n-type contact layer in a semiconductor laser. In short, the patent document 1 never provides a concrete and effective method or process for excluding the ripples occurred in the FFP in a semiconductor laser.
As explained above, even if a semiconductor laser is manufactured in faithful accordance with the method disclosed in the patent document 1 as a trial, there is no guarantee of obtaining the semiconductor laser which has no ripples in the FFP. Accordingly, it is quite difficult to obtain a semiconductor laser which has no ripples in the FFP and oscillates light in stable condition from the prior technique disclosed in the patent document 1.
When a contact layer formed at the lower part of a laser cavity is not removed, lights are leaked from that part (the contact layer), and that prevents the device from effectively oscillating a stable laser. When the contact layer formed at the lower part of the laser cavity is moved, electrical connection between the laser cavity and a negative electrode cannot be optimum, that prevents the device from providing an excellent distribution of electric current density. Accordingly, a device which oscillates a stable laser effectively cannot be obtained.
In the meantime, as described in the non-patent document 1, some of the ripples in the FFP may be suppressed by forming an n-type clad layer to have thickness of at least 0.8 μm or more.
By employing the method disclosed in the non-patent document 1, however, unless the n-type clad layer has thickness at least from 0.8 μm to 1 μm, although it depends on condition such as compositions of each semiconductor layers, e.g., an active layer, an optical guide layer and an n-type clad layer, the interference fringe (ripples in FFP) in the n-type contact layer cannot be suppressed sufficiently. Consequently, productivity of an n-type clad layer in its crystal growth process is poor in the conventional art shown in the non-patent document 1.
Because an extremely general method in which a peak intensity of the interference fringe is weakened in accordance with thickness of the n-type clad layer is employed in the non-patent document 1, the peak intensity of the interference fringe, which is occurred by lights leaked into the n-type clad layer, may be weakened, but it cannot be disappeared completely by employing the conventional method disclosed in the non-patent document 1.
And from the viewpoints of radiation effect, inner quantum efficiency, and threshold voltage which are related with a laser cavity, forming the n-type clad layer to have thickness as much as described above is not preferable with respect to the structure for a semiconductor laser.
As other method for confining lights, forming an n-type AlGaN layer in place of the n-type clad layer made of n-GaN may be employed. According to this method, because refractive index of AlGaN is relatively small, lights may be confined in a laser cavity.
However, a problem persists in even employing such a method. Because of inner stress which is generated between semiconductor layers owing to difference of lattice constants, forming a thicker film for confining lights becomes extremely difficult, and that makes it almost impossible to manufacture a semiconductor laser.
Alternatively, other method in which the refractive index of the n-type clad layer is suppressed by doping other impurities may be applied. But when so much amount of impurities are doped that the refractive index of the n-type clad layer remarkably changes, crystallity of the n-type clad layer and semiconductor layers deposited thereon may be remarkably deteriorated, which tends to become an outstanding problem. Accordingly, it becomes almost impossible to manufacture a semiconductor laser.